ABSTRACT
Indications of diapirism and soil circulation are common in periglacial areas, but governing mechanisms remain unclear. We explore the possibility that such motion may be driven, at least in part, by buoyancy forces that arise seasonally in thawing ice-rich soil. Because thawing proceeds downward from the ground surface, the duration of the thaw phase and, hence, the time available for progressive soil compaction decrease with depth. This leads to greater compaction and higher soil density near the ground surface but is partially offset by an increase in the rate of compaction and decrease in segregation ice volume with depth in the upper part of the active layer.
A theoretical model of thaw consolidation that parallels recent geophysical analyses of buoyancy-driven segregation of relatively light fluid from a viscous porous matrix provides quantitative insight into the soil compaction process. The model indicates that the density profile in a soil layer above the thaw front can be gravitationally unstable for much of the thaw season. Buoyancy can be a very effective driving force for diapirism where low permeability soil occurs at the base of the active layer. This is consistent with field evidence indicating that fine-grained soil commonly ascends to the ground surface in periglacial areas, particularly where groundwater conditions are favorable. On the other hand, buoyancy is generally incapable of driving wholesale soil circulation in unpatterned active layers but may incrementally contribute to long-term circulatory soil motion in established sorted circles.
